Solvent extraction is a chemical process by which two or more immiscible fluids, or a fluid and a solid, are mixed to transfer chemicals dissolved or suspended in one fluid or solid into another fluid. Metal and metalloid compounds can be separated from aqueous and solid media by extraction with appropriate solvents. This extraction procedure usually is, however, time and labor intensive. Furthermore, conventional solvent extraction often involves the use of toxic organic compounds, creating environmental problems in connection with the handling and disposal of spent solvent.
Recently, supercritical fluid extraction has become an attractive alternative to conventional solvent extraction for the recovery of metals and metalloids from liquid and solid media. For example, U.S. Pat. No. 5,356,538 to Wai et al., incorporated herein by reference, discloses a method for extracting metal and metalloid compounds from a solid or liquid material by exposing the material to a supercritical fluid containing a chelating agent. In particular, it teaches extracting metal ions from media using supercritical carbon dioxide and dithiocarbamate ligands. Other patent applications by Wal et al., describing different aspects of supercritical fluid extraction, also are incorporated herein by reference, including: FLUID EXTRACTION, application Ser. No. 08/253,000, filed Jun. 2, 1994, FLUID EXTRACTION, application Ser. No. 08/458,759, filed Jun. 2, 1995; EXTRACTING METALS DIRECTLY FROM METAL OXIDES, filed Nov. 3 1995, now U.S. Pat. No. 5,606,724; and FLUID EXTRACTION, application Ser. No. 08/692,894, filed Jul. 26, 1996.
.beta.-diketones, dithiocarbamates, trialkyl- and triarylphosphates and crown ethers have been used in the chelation and supercritical fluid extraction of metal compounds. Once the chelating agent, or ligand, has coupled to the metal, the resulting material is referred to as a metal-ligand complex. In many industrial processes, it is subsequently necessary to remove the metal-ligand complex from the supercritical fluid. Wai et al.'s U.S. Pat. No. 5,356,538 teaches removing the metal-ligand complex from the supercritical fluid by reducing the pressure, which results in the expansion and dissipation of the supercritical fluid and the deposition of the metal-ligand complex.
Although Wai's method-is an advance in extraction technology, one drawback of Wai's method is that an intact metal-ligand complex is recovered from the supercritical fluid. Wai et al.'s United States Patent Application entitled METHOD AND APPARATUS FOR BACK-EXTRACTING METAL CHELATES, application Ser. No. 08/692,894, filed Jul. 26, 1996, and incorporated herein by reference, teaches a method for separating the metal from the metal-ligand complex after extraction. This method involves back extraction of the supercritical fluid with an acidic solution. The metal is released from the complex into the acidic solution while the ligand remains in the supercritical fluid. While this method is useful when applied to industrial processes that require ligand recycling, the method uses caustic acid, a potentially harmful and environmentally toxic material. Furthermore, it is necessary to go through additional steps to separate the metal from the acid solution.
A method for forming fine particles using supercritical fluids has been described in U.S. Pat. No. 5,639,441 to Sievers et al (Sievers). Sievers discloses a method and apparatus for forming particles of a desired substance. Sievers' method involves dissolving the substance in a fluid, particularly water, to form a solution, and then mixing the solution with a supercritical fluid to form an immiscible system. The pressure is then reduced, typically quite quickly, such as by venting the system to atmospheric pressure as a spray. This creates an airborne dispersion or aerosol and forms fine particles.
A method to deposit films of a desired material from a supercritical fluid by reducing the pressure of the supercritical fluid has been described. U.S. Pat. No. 4,970,093 to Sievers et al. teaches a method for producing a film of a desired material by dissolving the desired material or a precursor into a supercritical fluid, rapidly expanding the supercritical solution, and then inducing a chemical reaction in the resulting vapor. This method therefore involves a chemical reaction that occurs after the supercritical fluid has expanded and vaporized.
It also is known that some metal-ligand complexes, such as copper-ligand complexes, can be dissociated by the addition of hydrogen in the presence of an organic solvent, such as kerosene. This reaction is an alternate means to dissociate the metal-ligand complex into its metal and ligand components. This process, however, has several disadvantages, including: (1) significant ligand degradation at the high temperatures required for the reaction to occur; and (2) the low solubility of hydrogen in organic solvents.